1. Field of the Invention
The present invention relates to an inspection apparatus and method, and a production method for pattern substrates and particularly to an inspection apparatus and method using transmitted light transmitted by a sample and reflected light reflected by the sample, and a production method for pattern substrates that uses the method.
2. Description of Related Art
In a semiconductor manufacturing process, if a pattern has a defect, a wire insulation failure or a short circuit may occur, thus reducing yields. Accordingly, an inspection apparatus is used to inspect whether a foreign object is sticking to a pattern substrate such as a semiconductor substrate and a photomask used in the semiconductor manufacturing process.
For such photomask inspection apparatuses, there are mainly two modes: a die-to-database mode and a die-to-die mode. In the die-to-database mode, an actually detected image and CAD data stored in a processor such as a computer are compared to detect a foreign object or a defect. However, in the die-to-database mode, for each pattern to be inspected, a reference image needs to be created from CAD data, and hence it is difficult to lower costs.
Meanwhile, in the die-to-die mode, images of occurrences of the same pattern located at different positions are detected, and by comparing them, the detection of a foreign object or a defect is performed. In this mode, a reference image for each pattern need not be created from CAD data, and hence costs can be lowered. However, with this mode, as to a mask having only one occurrence of the same pattern on its substrate, a foreign object or a defect therein cannot be detected.
In order to solve this problem, a method is disclosed where a transmission image and a reflection image are picked up by a camera to be overlaid is disclosed (refer to, for example, Japanese Unexamined Patent Application Publication No. Hei 9-311108). In this inspection apparatus, transmitted light and reflected light are combined to detect with a single detector. The intensity of light transmitted by a transmitting pattern of a mask and the intensity of light reflected by a light shielding pattern of the mask are adjusted. By this means, in a combined optical image (combined image), brightness is the same for pattern areas, where a light shielding is formed, and non-pattern (transmitting pattern) areas, where no light shielding is formed. In the transmitting pattern, transmitted light through a place to which a foreign object is sticking decreases in amount because transmittance decreases due to the foreign object. Meanwhile, in the light shielding pattern, reflected light from a place to which a foreign object is sticking decreases in amount because reflectance decreases due to the foreign object. Therefore, the detected amount of light from a place with a foreign object thereon decreases as compared with a normal place not depending on whether the place with the foreign object thereon is in the light shielding pattern or in the transmitting pattern.
Hence, by comparing the output of the detector and a foreign object detection threshold, foreign object inspection can be performed. With this configuration, a combined image of a transmission image and a reflection image is detected with a single detector, and hence the inspection apparatus can be simply configured.
Moreover, an inspection apparatus which picks up a transmission image and a reflection image separately for inspection is disclosed (refer to Japanese Unexamined Patent Application Publications No. 2004-354088 and No. 2006-72147). In this inspection apparatus, a transmitted illumination optical system and a reflected illumination optical system are each provided with a stop. A half of the field of view of an objective lens is illuminated with illumination light from the transmitted illumination optical system, and the other half is illuminated with illumination light from the reflected illumination optical system. Then, a transmission image formed by light transmitted by the photomask is picked up by a transmission image sensor, and a reflection image formed by light reflected by the photomask is picked up by a reflection image sensor. That is, the transmission image and the reflection image are received by separate sensors. Then, the output of each sensor and a threshold are compared for inspection. As such, both a transmission image and a reflection image are picked up, and hence the photomask can be accurately inspected.
This inspection apparatus will be described using FIG. 3. FIG. 3 diagrammatically shows the field of view of an objective lens on a photomask. The field of view 70 of the objective lens is circular, and a half thereof is a transmission illuminated area 71 illuminated by the transmitted illumination light from the transmitted illumination optical system, and the other half is a reflection illuminated area 72 illuminated by the reflected illumination light from the reflected illumination optical system. That is, the field of view 70 of the objective lens is divided into halves according to a straight line 77 passing through its center. One of the halves is the transmission illuminated area 71 and the other is the reflection illuminated area 72. Let a transmission image picking-up area 75 be an area picked up by a transmission image sensor and a reflection image picking-up area 76 be an area picked up by a reflection image sensor. The transmission image picking-up area 75 is included in the transmission illuminated area 71, and the reflection image picking-up area 76 is included in the reflection illuminated area 72.
In the above inspection apparatus, a detection optical system can be common, thus simplifying the configuration of the apparatus. Further, both transmission images and reflection images can be obtained by scanning the entire photomask once. That is, a transmission image and a reflection image can be picked up at the same time, hence shortening inspection time.
In order to further shorten inspection time of the above inspection apparatus, the transmission image picking-up area 75 and the reflection image picking-up area 76 are preferably enlarged in the field of view 70 of FIG. 3. By enlarging an area whose image can be picked up at one time, the scan distance becomes shorter, thus shortening inspection time. The transmission image picking-up area 75 and the reflection image picking-up area 76 are preferably enlarged as much as possible. Accordingly, the transmission image picking-up area 75 and the reflection image picking-up area 76 become closer to the straight line 77.
However, there is the following problem with such a defect inspection apparatus. If the transmission image picking-up area 75 and the reflection image picking-up area 76 are enlarged to shorten inspection time, the transmission image picking-up area 75 and the reflection image picking-up area 76 become closer to each other. That is, the transmission image picking-up area 75 and the reflection image picking-up area 76 are located adjacent to the straight line 77. In this state, if an illumination area deviates, illumination light from the transmitted illumination optical system may be incident partly on the reflection image picking-up area 76, or illumination light from the reflected illumination optical system may be incident partly on the transmission image picking-up area 75. For example, if the transmission illuminated area 71, on which transmitted illumination light is incident, spreads over the straight line 77, the transmitted illumination light is incident partly on the reflection image picking-up area 76. Thus, the reflection image picking-up area 76 is not uniformly illuminated, and hence the photomask cannot be accurately inspected.
Especially, there is variation in thickness of photomasks. To be specific, a variation of about ±100 μm exists with 6.35 mm thick photomasks. Thus, where the objective lens is placed on the pattern-formed-surface side of a photomask, if thickness varies between photomasks, the focal point of the transmitted illumination optical system deviates from the pattern formed surface. Furthermore, aberration occurs due to the difference in thickness of photomasks, and hence focusing becomes difficult.
For example, when an image of the view field stop of the transmitted illumination optical system is formed on the pattern formed surface, the image is blurred with variation in thickness of photomasks. Thus, the transmission image picking-up area 75 spreads. Hence, the transmitted illumination light is incident partly on the reflection image picking-up area 76, so that the reflection image picking-up area 76 is not uniformly illuminated. Therefore, there is the problem with conventional inspection apparatuses that accurate inspection cannot be performed in a short time.
The present invention was made in view of the above problem, and an object thereof is to provide an inspection apparatus and method which can perform accurate inspection in a short time, and a production method for pattern substrates.